1. Field of the Invention
The present invention relates to an image writing device using digital light-emitting elements such as a light emitting diode (LED) array.
2. Description of the Related Art
Generally, image writing devices that write latent images by irradiating light on a photo-sensitive element include a laser diode (LD) scanning method and a method of using an array of light-emitting elements in which LED elements are arranged in an array.
In the method of using an array of light-emitting elements, when a binary image is output by an image forming apparatus, one dot print can be printed in a rounded elliptical shape according to a process condition. In a case of one-dot lattice image (an image at 5-mm interval), this phenomenon appears clearly, and a vertical line is printed thicker than a horizontal line. This causes a problem of aspect ratio.
To solve the problem of aspect ratio, there is a method of controlling the problem by LED balance correction data. As a correspondence with multilevel data, tone data for each LED, data for correcting an output difference in a unit of block including a plurality of LEDs, and data for correcting an output difference with respect to a mean value of the block are added, and a difference in LEDs is suppressed according to the added data.
A conventional technique is described in Japanese Patent Application Laid-Open No. 2002-283609, in which writing devices formed of a plurality of LED heads (a light-emitting-element array unit) arranged, with an imaging position being shifted in a rotation direction of a photo-sensitive element, are provided in a staggered arrangement along an axial direction of the photo-sensitive element, image data to be transferred to the respective LED heads is divided by an LED write control circuit for each LED head based on an instruction from a printer control circuit, and the divided image data is transmitted to the respective LED heads, with the image data being shifted timewise in the rotation direction (feed direction) of the photo-sensitive element by a position to be imaged. Japanese Patent Application Laid-Open No. H6-270471 discloses a technique in which a divided exposure data transfer method is effectively used, to form a dot latent image on a photo-sensitive element, while appropriately lighting up LED elements arranged in a line multiple times in a sub-scanning direction for each one scanning line, and lighting frequency in the sub-scanning direction is controlled based on a correction value corresponding to a difference in an amount of light between the LED elements. Furthermore, Japanese Patent Application Laid-Open No. 2001-121745 discloses a technique in which a light-emitting element having a beam diameter in the sub-scanning direction different from an original beam diameter (a beam diameter as a reference) is lighted up multiple times with respect to one lighting signal with an intensity (an amount of light) different from an original intensity, with the position thereof being changed in the sub-scanning direction, so that a composite profile thereof is made to match an original beam diameter profile, thereby suppressing density nonuniformity caused by a difference in beam diameter in the sub-scanning direction for each light-emitting element.
A binary method includes a control by adding binary image data and correction data for each LED, to reproduce a tone with high fidelity. In this method, since dot printing power (print driving current control) is adjusted, line drawing is improved. However, vertical and horizontal line widths are not improved.
There are also methods in which the correction data and the printing image data are not added according to the LED array.
In a technique disclosed in Japanese Patent Application No. 2003-412065, control is performed by setting the lighting frequency for several times. In a light-emitting-element array unit in which after transferring even pixel data of image data, odd pixel data is transferred, the light-emitting elements are lighted up twice with one image transfer.
In a technique disclosed in Japanese Patent Application No. 2005-36620, data transfer is performed in a manner that even data and odd data are respectively transferred several times (twice). In a case of even data, pattern recognition is performed in a main-scanning direction at the time of data transfer for the second time, and in a case of an isolated vertex, data is converted from “1” to “0”. On the other hand, in a case of odd data, the same processing is performed for the first data, and a vertical line width is thinned by controlling the lighting time, thereby improving the vertical and horizontal line width.
In these conventional binary methods, control is performed for reproducing the tone with high fidelity by adding the binary image data and the correction data for an individual LED. However, since the dot printing power (print driving current control) is adjusted, the vertical and horizontal line widths are not improved.
Furthermore, in these conventional techniques, control with respect to a diagonal line image is not taken into consideration.